As quoted by YEC Woodmorappe (1999, p. 24), Dalrymple (1984, p. 101) admits that
many terrestrial lead samples do not EXACTLY lie on a 4.55 billion year old meteorite
isochron. However, Woodmorappe (1999, p. 24) fails to mention that these deviations
are trivial (also see Figure 8 in Dalrymple, 1984, p. 102). Specifically, the
uncertainty in the age of the Earth as derived from Pb isotope measurements is only about
2% (Dalrymple, 1984, p. 101).

When compared with meteorites, Earth rocks generally have much more complex
histories, which may include weathering from oxygen and water, melting, metamorphism
(heating without melting), faulting or other alteration events. To be exact, the Earth is
so dynamic that few terrestrial samples older than 3.8 billion years have been found (for
some details, see Dalrymple, 1991, chapter 4).

As explained by Dalrymple (1991, p. 117), nature cannot fractionate (that is,
separate and concentrate) 204Pb, 206Pb, 207Pb, and other lead isotopes from each other.
The atomic masses of the different Pb isotopes are too similar and their chemical
properties are identical. Unless YECs want to invoke groundless miracles or other
speculations, the only known way of varying the 206Pb/204Pb and 207Pb/204Pb ratios in
different rocks and minerals is through the radiometric decay of uranium OVER TIME.

Dalrymple (1991, chapter 7) also summarizes in great detail the various efforts of
different researchers to unravel the origin and histories of terrestrial leads. Now,
some YECs have accused Dalrymple (1991) of 'painting too rosy a picture' of radiometric
dating. However, Dalrymple (1991, chapter 7) discusses both the successes (e.g.,
Tera, 1981) and failures (e.g., the faulty assumptions in Ulrych, 1967) in interpreting
the geochronology of terrestrial leads. Despite Woodmorappe's (1999) misunderstandings
about the Precambrian history of the Earth and the functioning of geological processes,
Pb-isotope dating of meteorites, lunar samples and terrestrial rocks, as well as the great
complexity and destructive processes on the Earth's surface are entirely consistent with
the Earth being 4.5 billion years old (Dalrymple, 1991, chapter 7). At the same
time, these data utterly demolish young-Earth creationism.

In an attempt to refute Dalrymple's (1984) claims that terrestrial leads are close
to the meteorite lead-isotope isochron, Woodmorappe (1999, p. 24) quotes the following
statement from Harper and Jacobsen (1996, p. 1131-1132):

'It is widely believed that studies of lead isotopes in terrestrial samples provide
a well-determined age of the Earth (for an excellent review, see Dalrymple, 1991).
We show this to be incorrect, even though a roughly accurate answer is sometimes obtained,
but is not necessarily related at all to the formation of the Earth. Other widely
cited systems such as Rb-Sr, I-Xe, and Pu-Xe also do not date terrestrial accretion and/or
core formation in any well-defined sense.'

However, what are Harper and Jacobsen (1996, p. 1131-1132) really saying in this
quotation? Do they agree with Woodmorappe and claim that a 4.5 billion year old
Earth is an utter fantasy or are they saying that the age requires better resolution?
Also, why do Harper and Jacobsen (1996, p. 1132) endorse Dalrymple (1991)?

In reality, Harper and Jacobsen (1996, p. 1131) argue that a new 182Hf-182W method
can provide better resolution for dating the formation process of the Earth than
traditional lead-isotope methods:

'Here we present an overview and some experimental results for the newly-developed
182Hf-182W isotope system that, once fully calibrated, may be able to constrain the mean
age of the Earth PRECISELY [my emphasis].'

Also, Harper and Jacobsen (1996, p. 1150) admit:

'Age of the Earth estimates based on Pb and Sr isotopes are not likely to provide
reliable information on the terrestrial accretion interval.'

In other words, Harper and Jacobsen (1996, p. 1131) correctly argue that
the Earth did not simply appear from nowhere 4.5 billion years ago, but that it formed in
stages over 100,000 to 10 million years, and that Pb isotope dating cannot provide
adequate details on this accretion period. Dalrymple (1991, p. 346-347) clearly agrees
with this view, except that he thinks the Earth took 20 million years or so to accrete.
Harper and Jacobsen (1996, p. 1131) further explain that because the Earth accreted or
grew over 100,000 to 10 million years, its 'age' is not clear-cut:

'The term "age of the Earth" is often employed loosely and may
refer to three different senses. First, it can be defined as the age of the solar
system, specifically obtained as the time of formation of the oldest known accreted
objects at 4566 +/- 2 Ma ...[reference omitted]. This can be justified because
models of planetary accretion imply rapid initial material coagulation processes leading
to runaway accretion of proto-planetary "embryo" nuclei on a timescale of only
0.1 Ma ... [reference omitted]. A second commonly used definition refers to the time
of the "end" of the Earth's accretion, that is to say to some time at which the
Earth had grown to very near its present mass. Here the answer depends on the exact
definition of "very near", because the accretion process had a long tail and is
technically still continuing today. [new paragraph] A third usage, the "mean
age" represents the mass-average age of accretion.'

Like Harper and Jacobsen (1996), Dalrymple (1991, p. 346-347) is well aware that
detailed interpretations of Pb-isotope dates are somewhat uncertain:

'As Tera [1981] observed, it is probably significant that ancient galenas from three
continents seem to define a common source with a common age and lead composition, and also
that the age obtained is similar to the age determined for meteorites. It is also
satisfying that the model ages calculated for the galenas by the congruency result are
within a few percent of the ages measured by other dating methods ...[reference to table
omitted]. The precise nature of the event of 4.53-4.54 Ga [billion years ago] is not
entirely clear, but it is Tera's opinion that the age represents the time of U-Pb
fractionation in the primary materials from which the Earth was formed. If this
fractionation occurred at the time the Earth accreted, then the age is the age of the
Earth; if not, then it represents the age of the debris from which the Earth was formed.
Alternatively, the fractionation might be the result of separation of the Earth's
materials into core and mantle. Regardless of the precise interpretation, the near
equivalency of this "age of the Earth" and the ages of the primitive meteorites
indicates that the condensation of solid matter from the Solar Nebula, the formation of
the meteorites' parent bodies, and the formation of the Earth as a planet occurred within
a period of only 20 Ma [million years] or so.'

Obviously, Woodmorappe (1999, p. 24) has misrepresented Harper and Jacobsen (1996,
p. 1131-1132) when he claims that this paper indicates that 'any' agreement between Pb
isochrons and the 4.5 billion year old age of the Earth is only 'coincidental.'
Considering Harper and Jacobsen's comments about the strengths and great potential of the
Hf-W method to provide better details on the early formation of the Earth, including core
formation, and how short-lived radioisotopes, such as 129I, form in supernovae and quickly
die during the long history of an ancient Universe, this article provides no comfort to
YECs like Woodmorappe.

Woodmorappe (1999, p. 24) also quotes Gariepy and Dupre (1991, p. 216-217), which
supposedly is critical of terrestrial leads supporting a 4.5 billion year old date for the
Earth:

'It was then thought that the Pb ores in these large deposits were derived from the
mantle and the lower crust and evolved in a closed system. However, most lead ores
of the world are 'anomalous' in that they do not fit a single-stage growth curve... a
single stage evolution since 4.5 Ga [billion years ago] is unlikely.'

Like Harper and Jacobsen (1996, p. 192), Gariepy and Dupre (1991) argue
that Pb-isotope dating of terrestrial samples is not sensitive enough to derive details on
the Earth's formation because most terrestrial leads have had long and complex histories
(also see chapter 7 in Dalrymple, 1991, which discusses the meaning of single and multiple
stage leads). Nevertheless, the 4.5 billion year old date is basically correct, as
stated in Gariepy and Dupre (1991, p. 192):

'In 1956, using data gathered on meteorites and on modern sediments considered
representative of the Earth, Patterson evaluated this age at 4.5 Ga [billion years old], A
FIGURE THAT HAS LARGELY BEEN CONFIRMED SINCE THEN [my emphasis].'

Here are some other statements by Gariepy and Dupre (1991, p.191, 192), which
hardly support Woodmorappe's (1999, p. 24) crusade:

'The variations in the abundance of lead isotopes provide a USEFUL method of
constraining a range of geological problems because i) it provides age information arising
from the radioactive decay of U, Th and their daughter products into stable Pb isotopes,
and ii) it retains a time-integrated record of U/Pb and Th/U ratios of the reservoir in
which the Pb had developed. These reservoirs vary in scale and may embody the
mantle, the continental crust, an ore deposit, a basement complex, a lithostratigraphic
unit or the atmosphere-hydrosphere system. Thus, a reservoir may be defined as any
geochemical unit of the Earth exchanging matter and energy with its environment. In
the context of this chapter, reservoirs are those parts of the Earth that operate as a
[sic] sources of Pb and possess distinctive U/Pb or Th/Pb values. These ratios are
fractionated significantly during partial melting, fractional crystallization, regional
metamorphism and hydrothermal circulation. They are also affected in complex ways by
weathering, biological activity and other low temperature processes occurring near the
surface of the Earth. As a result, the isotopic abundances of Pb vary widely in
nature, depending on the age and the geological history of a given reservoir. [new
paragraph] In contrast to the isotopes of light elements, the isotopes of Pb
are not fractionated from one another during physico-chemical processes such as
dissolution, metal transportation and precipitation.' [my emphasis]

'The chapter does not propose a unified model of Earth's evolution integrating all
available geochemical and geological information, but rather attempts to point out how Pb
isotopes CAN BE USED to decode certain aspects of terrestrial evolution and constrain some
key parameters.' [my emphasis]

Finally, in contrast to Woodmorappe and other YECs, Gariepy and Dupre
(1991, p. 224) argue:

'Although this chapter is focused on Pb isotopes as tracers of crust-mantle
evolution, it should be emphasized that the combined use of several isotopic tracers is an
EVEN MORE POWERFUL tool without which some of the remaining uncertainties will not be
resolved.' [my emphasis]

CONCLUSIONS

The uncertainties over the age of the Earth raised by Dalrymple (1991), Harper and
Jacobsen (1996) and Gariepy and Dupre (1991) are actually trivial (typically, 4.52 to 4.56
billion years, Dalrymple, 1991, p. 356) and provide no comfort to YECs. Terrestrial leads
are very close to the 4.55 billion year old meteorite-based isochron and, as Dalrymple
(1984, p. 101) points out, leads in Moon rocks also lie very close to the line and
indicate a 4.5 billion year old age for the Moon. Besides misrepresenting the
contents of Harper and Jacobsen (1996), Dalrymple (1984, 1991) and Gariepy and Dupre
(1991) on the age of the Earth, Woodmorappe (1999, p. 24) produces no scientific
references to attack lead isotope studies that clearly support an ancient origin for
meteorites and Moon rocks. The only references that Woodmorappe (1999, p. 24)
provides to supposedly attack lead isotope dates from meteorites are Witter (1974) and
Williams (1992, p. 2), who are YECs and not geochronologists.

REFERENCES

Dalrymple, G. B., 1984, 'How Old is the Earth?: A Reply to "Scientific"
Creationism', in Proceedings of the 63rd Annual Meeting of the Pacific
Division, American Association for the Advancement of Science, v. 1, pt. 3, Frank
Awbrey and William Thwaites (Eds).

Dalrymple, G.B., 1991, The Age of the Earth, Stanford University Press,
Stanford, California.